1. Field of the Invention
The present invention relates to a suspension device for line-mounted surge arresters (hereinafter referred to as line arresters) for use in air-insulated power transmission lines in high-voltage transmission or distribution networks, that is, in electrical power systems with voltages from a few kilovolts up to several hundred kilovolts. The present invention also relates to a method in and use of such a suspension devices for line arresters.
2. Discussion of the Background
Line arresters are a type of arresters which are used for preventing overvoltages, such as switching overvoltages or overvoltages caused by lightning strokes, from propagating in air-insulated transmission lines. If it reaches a switchgear installation, an overvoltage may damage electrical apparatus or force circuit breakers to trip and thus destroy parts of the electrical power system. To prevent this, line arresters are connected, at regular intervals, to the conductors of the transmission line.
A surge arrester exhibits a non-linear current-voltage characteristic in that its conductivity increases non-linearly with the voltage. A modern surge arrester comprises series-connected blocks of substantially zinc oxide which are enclosed in a porcelain or polymer insulator. At normal voltages across the blocks, their conductivity is practically zero, but at high voltages the blocks become electrically conductive.
The line arrester owes its name to the fact that it is connected to the electrical power system out on the line, that is, in the transmission line. The line arrester has an elongated shape and is connected at one end to one of the conductors of the transmission line and at its other end to ground potential. The voltage across the line arrester is thus equal to phase voltage. At normal phase voltage, the conductivity of the line arrester is, in principle, zero, but when an overvoltage arises on the line, the line arrester becomes conductive and the overvoltage is conducted to ground. An overvoltage is thus prevented from propagating further in the transmission line. When the voltage of the conductor again drops to normal level, the conductivity of the line arrester returns to zero.
Admittedly, a line arrester is dimensioned to handle overvoltages but it does not withstand any amount of overvoltages. There is a risk that the line arrester, after an overload, that is, a voltage load which is higher than that for which the line arrester is designed, will have a current-voltage characteristic which deviates from the original one. It is possible, for example, that the line arrester continues to be electrically conductive after the voltage across it has returned to normal level. To prevent the line arrester from disturbing the electrical power system, it is therefore important that it is disconnected as rapidly as possible from the transmission line after an overload. After the line arrester has become overloaded, it is expended and must be replaced.
In an air-insulated transmission line, the high-voltage conductors are supported, via insulators of porcelain or glass, by transmission towers. If line arresters are used in the transmission line, the arresters are connected to the conductors in the vicinity of these transmission towers. Each line arrester may then be grounded by being connected to the nearest tower. A condition for this to function is, of course, that the towers themselves are grounded and that they are made of steel or any other electrically conductive material. The line arrester is mounted at a transmission tower by means of a suspension device comprising a first and a second suspension part. Via the first suspension part, the first end of the line arrester, the high-voltage end, is connected to the high-voltage line, and via the second suspension part the second end of the line arrester, the ground end, is connected to the transmission tower. The high-voltage conductor is in electrical contact with ground potential, via the line arrester and the tower, and is thus parallel-connected to the insulator which insulates the high-voltage line from the transmission tower. At an overload of the line arrester, this contact must be broken. Some of the two suspension parts is, therefore, provided with a disconnecting device which is released at an overload and hence physically breaks the electrical contact between the high-voltage conductor and the transmission tower. One example of a frequently used disconnecting device is a blasting cap which explodes when subjected to high powers.
The location of the disconnecting device depends on how the line arrester is mounted at the transmission line. The line arrester may, for example, be mounted such that, via the first suspension part, it is suspended vertically from one of the conductors of the transmission line and is connected to the transmission tower via the second suspension device, which in this case consists of a ground line. In such a mounting arrangement, the disconnecting device is usually placed where the ground line is connected to the ground side of the line arrester. When the line arrester is overloaded, the connection of the high-voltage line to ground is broken by the disconnecting device being released, whereby the ground conductor, under the influence of its own weight, falls and remains hanging from the transmission tower.
However, the present invention deals with line arresters where, contrary to the example above, the longitudinal axis of the line arrester deviates from the vertical line when the line arresters are in operation. In such a mounting arrangement, the disconnecting device is placed in one of the two suspension parts, and in the other of the two suspension parts the line arrester is articulately attached. At an overload of the line arrester and a subsequent release of the disconnecting device, the line arrester, under the influence of its own weight, performs a rotating movement around that suspension part from which the line arrester is articulately suspended, whereby the line arrester swings back and forth past the vertical line before it finally adopts a vertical position. Because of the swinging motion, that suspension part, from which the line arrester is articulately suspended, will be subjected to such a mechanical load that it runs the risk of breaking, causing the line arrester to fall to the ground. Since the line arrester in most suspension arrangements is suspended in the vicinity of an insulator and a transmission tower, these parts run the risk of being hit by the swinging line arrester if the amplitude of the swinging motion is great. A powerfully swinging motion thus entails a risk of the line arrester damaging the insulator or the transmission tower. In addition, there is a risk that the line arrester itself is broken against the insulator or the transmission tower, whereby parts of the line arrester will fall to the ground.
The object of the presents invention is to provide a suspension device for line arresters in which the above-mentioned disadvantages of a swinging line arrester are avoided by controlling and damping the motion of the line arrester. This is achieved according to the invention with a suspension device comprising two suspension parts, in which one of the suspension parts comprises a damping member which, when the disconnecting device is released, influences the line arrester with a force which is directed against the swinging motion and which damps the motion during at least part of the motion, whereby the kinetic energy which the line arrester possesses is transformed into thermal energy in the damping member or on the surface thereof. In this process the damping member may be allowed to be deformed plastically or elastically.
In most suspension arrangements, the line arrester may be allowed to swing with an amplitude which deviates 15xc2x0 from the vertical line without a risk of the line arrester hitting a transmission tower or an insulator. The damping member should therefore be dimensioned such that the turn angle, that is the angle between the vertical line and the line arrester when it turns in its movement, is not larger than 15xc2x0. Preferably, however, the damping member is dimensioned such that the turn angle is not larger than 5xc2x0 to prevent the load on that suspension part, on which the line arrester rotates, from becoming too great.
For each line arrester, the damping force, and hence the damping member, must be adapted to the weight and the length of the line arrester and to the angle between the longitudinal axis of the line arrester and the vertical line during normal operation, that is, the initial position of the line arrester. The damping member must also be adapted to the maximum permissible turn angle permitted by the suspension arrangement.